1. Field of the Invention
This invention generally relates to shielding of a dense package in a high frequency environment. The invention further relates to isolation and impedance matching for electronics. Specifically, the invention is a group of resistors mounted in a high density network or array package with a ground plane that uses solder balls to connect with a circuit board.
2. Description of the Related Art
Resistor networks are commonly used to terminate high speed digital signal lines to minimize unwanted reflections back through the transmission structure by matching the impedance of the transmission structure. In most applications, the terminations are made by placing a resistor matching the impedance of the transmission line at the end of the transmission line. One end of the resistor is connected to a common termination voltage and the other end is connected to the signal line.
One problem that can occur in electronic packages is that electromagnetic energy may be coupled between adjacent conductors such as circuit lines. The adjacent conductors may be above each other or may be side by side in the same plane. The circuit lines may be next to each other on the same plane or may be coupled in between different planes. For example, electromagnetic energy can be coupled from a printed circuit board to a component mounted above the board and vice-versa.
In high speed, high density circuits, it is desirable to minimize reflected signal energy due to impedance mismatches in order to improve signal integrity and have low noise induced on circuit lines located in close proximity. Impedance is the opposition that an electronic component, circuit, or system offers to alternating and/or direct electric current flow. The impedance has two components: a real component and an imaginary component. Impedance is a vector quantity that is measured and quantified as resistance and reactance.
Resistance is a measure of the extent to which a substance opposes the movement of electrons among its atoms. The more easily the atoms give up and/or accept electrons, the lower the resistance. Resistance is expressed by a positive real number measured in ohms. Resistance can be measured with alternating current (AC) and also with direct current (DC).
Reactance is an expression of the extent to which an electronic component, circuit, or system stores and releases energy as the current and voltage fluctuate with each AC cycle. Reactance is expressed by an imaginary number in ohms. It is observed for AC current, but not for DC current. When alternating current passes through a component that contains reactance, energy might be stored and released in the form of a magnetic field, in which case the reactance is inductive (denoted+jXL). In another case, energy might be stored and released in the form of an electric field, in which case the reactance is capacitive (denoted−jXC). The formula for inductive reactance is XL=2 πfL. Where L is inductance and f is frequency. The formula for capacitive reactance is XC=2 πfC. Where C is capacitance and f is frequency.
Reactance is multiplied by the positive square root of −1, which is the unit imaginary number called the j operator, to express impedance (Z) as a complex number of the form R+jXL (when the reactance is inductive) or R−jXC (when the reactance is capacitive). For example, if a resistance of 100 ohms is connected in series with an inductance of 10 nh and operated at 4.0000 MHz, the impedance is given by ZRL=R+jXL, where X is given by 2 πf and f is frequency. Multiplying the terms gives ZRL=100.00+j.251.
In order to have low cross-talk noise and minimize reflections, It is desirable to have a low reactive element of the resistive component of the impedance approximately the same as the resistance of the resistor network. For low resistance values, the reactive part of the impedance is due to the parasitic capacitance and inductance of the resistor package and typically results in a reactance that is above the resistor value. Due to space constraints and the dielectric constants of the substrate materials used, it is difficult to achieve a well matched impedance value in present resistor networks.
Therefore, there is a current unmet and heretofore long felt need for a resistor network that has low cross-talk noise, improved signal integrity, improved shielding and can be designed with a range of matching impedances.